MDEV-33672 (10.6) added checks/tests for malformed events which end
before the flags describe (which would lead to reading of un-owned
memory). MDEV-7850 (11.5) extended all GTID events with a thread id
at the end of the event. This GTID event extension invalidates the
tests added in MDEV-33672 because the thread id is appended after the
event (and thereby the event isn't cut short).

This patch fixes these MDEV-33672 tests by not writing the GTID
thread id when writing the Gtid events just for these tests. This
preserves tests for backwards compatibility, rather than getting rid
of the tests altogether.